The field of the invention is the field of purifying water by removing heavy metal ions. In particular, the field is eliminating nickel and lead ions from iron(II) salt solutions.
Heavy-metal ion concentrations in solutions are generally reduced (ie the solution is purified) by chemical precipitation of the corresponding hydroxides or carbonates. On the industrial scale, however, the purification effect expected on the basis of the low solubility product is often not achieved for a variety of reasons. The influencing factors are the time (incompleteness of the reaction), the content of other substances present in the solution (their xe2x80x9cionic strengthxe2x80x9d), possible complex-forming reactions (conversion into compounds of higher solubility) and, above all, the handling properties of the precipitates (e.g. their filterability).
The precipitation of heavy-metal ions in the form of heavy-metal sulfides is generally also possible, and sometimes also preferred, since the solubility of the sulfides is often sufficiently low even in the acidic range (see, for example, B. D. Bhattacharyya et al., Precipitation of heavy metals with sodium sulfide: Bench-scale and full-scale experimental results, American Institute of Chemical Engineers Symposium Series 209, Vol. 77 (1981) 31-38). However, it is pointed out in L. Bauch et al., Elimination von Blei und Nickel aus salzreichen Wxc3xa4ssern durch Sulfidxc3xa4llung und Flockung {Elimination of lead and nickel from salt-rich waters by sulfide precipitation and flocculation}, Vom Wasser, 75, 375-392 (1990) that the low residual concentrations to be expected on the basis of the thermodynamic data are virtually unattainable, particularly for nickel in acidic salt-rich waste water. No prior art documents show substantial removal of nickel and lead sulfide precipitates under such conditions.
It is an object of the invention to produce an industrial-scale method for the extensive elimination of lead and nickel ions from iron salt solutions, where the iron ions are predominantly in bivalent state. In this context, xe2x80x9cextensivexe2x80x9d is taken to mean that a value below 30 mg per liter is reliably achieved for nickel and below 7.5 mg per liter for lead, but that as little dissolved iron as possible is precipitated at the same time.
The object is solved by a method for the elimination of nickel and lead ions from iron(II) solutions by precipitation and separation of the corresponding sulfides, characterised in that an alkali sulfide is added at a super-stoichiometric rate in the acidic range and the precipitate filtered off after a residence time.
The invention is a method for the elimination of nickel and lead ions from high concentration iron(II) acidic solutions by adding an alkali sulfide, waiting a residence time, and filtering off the nickel and lead precipitate.